Fluorine generating cells produce both gaseous fluorine and hydrogen by the electrolysis of hydrogen fluoride. Fluorine and hydrogen explosively recombine when they contact each other, therefore, it is necessary to keep them completely separate when generated during electrolysis. Fluorine cells are generally constructed such that the two gases are collected in two separate compartments above the surface level of the electrolyte. The compartments are often separated by means of a so-called skirt, the skirt often being part of and depending from an upper, generally horizontal wall of the cell, and extending into the electrolyte and surrounding the anode. However, the skirt must remain electrically neutral with respect to the anode which it surrounds and to the cathode which is often formed by the inner wall of the cell container vessel. It is, therefore, necessary that the skirt is electrically insulated from the anode (and cathode) and for the anode connection (often referred to as the “stud”) to pass through the skirt or be connected thereto whilst completely sealing the fluorine compartment against leakage of fluorine.
It has been the practice to have a stud for connection to the anode and which stud passes through an insulating member and seal, often made of plastics material such as a fluoroelastomer rubber, in the upper horizontal skirt wall portion. However, due to the high currents which are inherent in fluorine generation by electrolysis a considerable amount of heat is generated by resistance heating, this condition often being exacerbated by poor electrical connection between the stud and carbon anode which is generally used. The effect of this heating can be to cause a runaway chemical reaction between the plastics seal material and the fluorine with which it is in direct contact and which may result in a fluorine leak. In extreme cases even the stud metal may itself burn in the fluorine gas stream in the resulting leak causing a so-called “stud fire”. This has been somewhat alleviated by the construction shown in WO 96/08589 where, instead of the anode connecting stud passing through an insulating seal in the skirt wall, the stud is formed by welding stud members on either side of the skirt wall, so that there is no through hole, and closing the fluorine compartment by an insulating gasket remote from the anode stud. However, even in this construction, whilst an improvement on earlier constructions, the sealing gasket is still directly contacted by the fluorine gas and is still susceptible to attack especially when there is an unexpected temperature rise for any reason.
In the case of the semiconductor industry, for example, it is essential that any process plant utilising toxic or hazardous gases such as fluorine, for example, possesses the most stringent levels of leak tightness since the majority of people working in such plants generally wear only normal non-protective clothing.